Grass Seed Calculator

How this is calculated

The arithmetic is simple: area divided by 1,000, multiplied by the rate in pounds per 1,000 square feet. Everything difficult about this is choosing the rate. TYPICAL NEW-LAWN RATES, in pounds per 1,000 square feet: tall fescue 8 to 10, perennial ryegrass 7 to 8, Kentucky bluegrass 2 to 3. OVERSEEDING AN EXISTING LAWN uses roughly HALF that: tall fescue 4 to 6, perennial ryegrass 3 to 4, Kentucky bluegrass 1 to 2. The goal there is to thread new grass into an existing canopy rather than to establish one from bare soil, and over-applying smothers what you already have. ⚠️ BLUEGRASS GOES DOWN AT ABOUT A QUARTER THE WEIGHT because its seeds are tiny - roughly 2.2 million to the pound against about 230,000 for tall fescue, nearly ten times as many in the same weight. Rates are quoted by weight because weight is how seed is sold, not because weight is the thing that matters. But the rates are NOT set to put the same number of seeds down, and it is worth being precise about this. Tall fescue at 9 pounds per 1,000 square feet is about 2,070 seeds per square foot. Kentucky bluegrass at 2.5 pounds is about 5,500 - roughly two and a half times MORE, on a quarter of the weight. That holds across the whole published range. The reason is that bluegrass establishes differently. Its seedlings are very small and slow to get going, and a mature bluegrass lawn fills in by spreading through rhizomes rather than by every seed becoming a plant. It is sown thickly and thins itself out. Fescue is a bunch grass that largely does not spread, so each seed has to count. Either way the practical rule is the same: the rate belongs to the SEED, not to the lawn. Applying a bluegrass rate to a fescue bag under-seeds badly; applying a fescue rate to bluegrass buries it. ⚠️ USE THE RATE ON THE BAG. Mixes are common, and a mix's rate is not the average of its parts. The bag is written for the exact blend inside it, which no general table can be. MORE IS NOT BETTER. Over-seeding produces dense, spindly seedlings competing with each other for light, water and nutrients, and the stand that results is weaker than a correctly seeded one. This is one of the few garden calculations where deliberately over-ordering is a mistake rather than insurance. What this does not cover, and what actually decides whether the seed grows: soil contact, timing, and water. Seed sitting on top of thatch does not germinate. Most cool-season grasses go down in early autumn, not spring. And new seed needs consistent moisture for weeks - missing that is what kills far more lawns than a slightly wrong rate.

Common questions

Why is the bluegrass rate so much lower?

Because the seeds are tiny. Kentucky bluegrass runs around 2.2 million seeds to the pound; tall fescue is roughly 230,000 - nearly ten times as many in the same weight. Rates are quoted by weight only because weight is how seed is sold. What the rates do NOT do is equalise the seed count, and it is worth being exact about that: fescue at 9 pounds per 1,000 square feet is about 2,070 seeds per square foot, while bluegrass at 2.5 pounds is about 5,500 - roughly two and a half times more, on a quarter of the weight. Bluegrass is sown thickly on purpose. Its seedlings are small and slow, and a bluegrass lawn fills in by spreading through rhizomes rather than by each seed becoming a plant. Tall fescue is a bunch grass that barely spreads, so each of its seeds has to count. The practical rule survives either explanation: the rate belongs to the seed, not to the lawn.

Should I use the table or the bag?

The bag, always. It is written for the exact blend inside it, and mixes are extremely common - a rate for a mix is not the average of its components' rates. The table in the notes here is for when you are planning before buying, or checking whether a bag's claim looks sane. Once you have the bag in your hand, its figure supersedes every general table including this one.

How much less seed for overseeding?

Roughly half the new-lawn rate. Tall fescue around 4 to 6 pounds per 1,000 square feet, perennial ryegrass 3 to 4, Kentucky bluegrass 1 to 2. The job is different: you are threading new grass into an existing canopy rather than establishing one on bare soil, so there is less room and less light available for each seedling. Applying a full new-lawn rate over living turf smothers what you already have.

Is extra seed insurance?

No - this is one of the few garden calculations where over-ordering actively hurts. Too much seed produces dense, spindly seedlings competing with each other for light, water and nutrients, and the resulting stand is genuinely weaker than a correctly seeded one. Buy the bags you need, apply the stated rate, and keep any surplus sealed for patching rather than spreading it because you have it.

What actually decides whether it grows?

Three things this calculation says nothing about, and all of them matter more than the rate. Soil contact - seed sitting on top of thatch will not germinate, so rake or scarify first. Timing - most cool-season grasses go down in early autumn rather than spring, when soil is warm and weed competition is falling. And water - new seed needs consistent moisture for weeks, and missing that kills far more new lawns than a slightly wrong rate ever has.

How long does leftover seed keep?

Not indefinitely. Germination rate falls year on year, and seed stored warm or damp falls off faster. Sealed, cool and dry, a leftover bag is usually still usable the following season for patching, but treat a bag that has been open in a shed for two years as unreliable rather than free. If you are seeding a whole lawn, that is not the place to find out.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Grass Seed Calculator at https://www.bfcbrilliance.com/tools/grass-seed-calculator.

What I entered:
- Length (ft): ___
- Width (ft): ___
- Seeding rate (lb per 1,000 sq ft): ___
- Bag size (lb): ___

What it calculated:
- Seed needed: ___
- Bags to buy: ___
- Area: ___
- In thousands of square feet: ___
- One bag covers: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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